Push-pull photocell circuit



Jan. 7, 1941. J. L. UNDERHILL 2,227,562

PUSH-PULL PHOTOCELL CIRCUIT Filed July 28, 1959 19+ 70 PHOHQCEZA a 70 Pl/070CEL z. 2

Imnentot Gttorneg Patented Jan. 7, 1941 PATENT OFFICE PUSH-PULLPHOTOCELL cmcorr Joseph L. Underhill, Riverton, N. J., assignor to Radio Corporation of America, a corporation of Delaware.

Application July 28, 1939, Serial No. 287,068 In Great Britain August 19, 1938 2 Claims.

In a push-pull photocell circuit adjustment is usually provided for varying the relative potentials applied to the two photocells for the purpose of balancing their outputs. This has been 5 clone in such a manner that the potential of one cell remains constant at a potential value somewhat lower than the rated maximum potential for such a cell, whilst means are provided for varying the potential of the other cell either above or below that of the first. Alternatively,

variation has been accomplished in such a manner that as the potential on one cell is increased, the potential on the other is decreased. In average operation of such known circuits, the efiiciency of the circuit is below optimum because in neither arrangement is: it possible, except under unusual conditions, to operate the less sensitive cell at its maximum rated potential and reduce the potential of the more sensitive circuit as necessary.

By utilizing the present invention, it is possible by a single adjustment to obtain maximum efficiency from the circuit by maintaining a maximum predetermined potential on either cell while reducing the potential on the other as desired.

According to the invention, a variable potentiometer having a fixed center tapping is provided with a sliding contact connected electrically with said fixed center tapping.

A further feature of the invention is the use of such a variable potentiometer in a circuit for controlling voltages for push-pull photocell circuits, thermionic valves and similar purposes.

One form of the arrangement is illustrated by the accompanying diagrammatic drawing.

A double potential dividing circuit is connected at a with the positive terminal of a potential source of somewhat higher potential than the maximum rated potential for each photocell;

b and 0 represent connections with two photocell circuits through which the positive potential is applied to those circuits. d, e, f and g are resistances of suitable values, h is a potentiometer of suitable resistance value having a sliding contact a which is connected with the negative side of the potential source at o. This potentiometer it has a fixed center tapping k which divides its total resistance into two equal parts m and n and which is connected with, but does not move with, the sliding contact a.

The values of the resistances. d and f, e and g, and the potentiometer h are selected so that the maximum predetermined potential will be 5 5 taken off between b and :i, and between 0 and when the sliding contact 9 is in the center position opposite the fixed contact 70. The values of these can be determined by the following formulae:

Desired maximum potential at b:

and the desired maximum potential at c:

where A represents the positive potential at the point a. If the sliding contact a be moved to the left, the potential at c will not vary, but the potential at the point I will be reduced. Alternatively, if the sliding contact 7' be moved to the right, the potential at b will not vary, but the potential at 0 will be reduced. In this manner it is possible to maintain the potential at either b or c at a maximum pre-determined value while reducing the potential at the other. The resistances j and g are not essential to the operation of the circuit and are inserted in order to provide a fixed minimum value to which the potential at either I) or c may be reduced. Without these the potential at either could be reduced to zero with respect to the side of the potential source connected with the sliding contact 9'.

Although the invention relates particularly to the balancing of photocell circuits, it might also be applied to other circuits such as the adjusting of the screen or grid voltages on two vacuum tubes. Furthermore, the same circuit could be utilized for other purposes in adjusting two potentials in such a manner that the potential of one of the two circuits is always maintained at a predetermined minimum. This could be done by utilizing the potential difference between a and b, and a and 0 instead of between o and b and o and 0.

Furthermore, although in the foregoing description the circuit is described as being connected at a with the positive terminal of a voltage source and at o with the negative terminal of said source, in some applications it may be desirable to utilize such a circuit with a connected with the negative terminal and o with the positive terminal of said source. In such an arrangement b and 0 could be maintained at a predetermined negative maximum with respect to 0 or a predetermined positive minimum with respect to a.

I claim as my invention:

1. A voltage controlling circuit including a bridge circuit having four arms, a source of potential connected to opposite ends of the bridge, potential responsive devices connected between opposite sides of the bridge and one end thereof, and a potential control device including a movable contact connected to the said one end of the bridge and movable selectively along either of the adjacent arms, one connection of said source of potential to said bridge being at the midpoint of said potential control device, whereby the voltage applied to the device connected to the arm along which said contact is moved is reduced without materially affecting the potential applied to the other device.

2. A photocell controlling circuit including a bridge circuit having four arms, a source of. potential connected to opposite ends of the bridge, photoelectric devices connected between opposite sides of the bridge and one end thereof, and a potential control device including a movable contact connected to the said one end of the bridge and movable selectively along either of the adjacent arms, one connection of said source of potential to said bridge being at the midpoint of said potential control device, whereby the voltage applied to the device connected to the arm along which said contact is moved is reduced without materially afiecting the potential applied tn the other device.

JOSEPH L. UNDERIEIL. 

